The invention relates to machines for measuring force variation or imbalance forces of rotating tires. More particularly, the present invention relates to a high speed tire testing machine including special damping means to reduce measurement errors due to mechanical resonance of the machine.
Known tire testing machines generally include a large diameter roadwheel rotatably mounted on a horizontal axis and a motor for rotating it. A tire spindle, to which the tire to be tested is mounted, is supported opposed to the roadwheel mounted in bearings. In order to generate electronic signals correlated with forces transmitted from the tire through the tire spindle, the machine includes a load cell assembly. That assembly comprises a load cell array disposed in force sensing relation between the tire spindle bearings and a load cell mount which is supported upon a rigid support structure. The load cells of the array are oriented to measure reaction forces between the tire and roadwheel and the tire along directions of interest which usually include three mutually orthogonal axes and two moments, the latter of which correspond to rocking of the tire spindle in vertical and horizontal planes, respectively. A reciprocable mechanism such as piston and cylinder is connected to the tire/tire spindle/load cell mount assembly to press the tire against the roadwheel with force sufficient to establish a desired radial load on the tire as it rotates in contact with the roadwheel and to retract the tire from engagement with the roadwheel upon completion of the test.
During testing, the roadwheel is driven to cause the tire to rotate at speeds corresponding to road speeds of up to 200 mph or higher while the load cells measure the forces of the tire as it rotates. In order to test at these speeds, a typically sized passenger car tire must rotate on the machine at a fundamental angular frequency of up to about 50 Hz. The machine must not only be capable of carrying out accurate force measurements not only at that fundamental tire rotational frequency but also at higher order harmonics thereof.
Every testing machine is characterized by one or more natural resonant frequencies usually including separate, although typically closely spaced, natural frequencies associated with both the roadwheel assembly and the structure supporting the combined tire/tire spindle/load cell mount assembly, respectively, both of which are typically about 50-60 Hz. In addition, the tire/tire spindle/load cell mount assembly itself has a characteristic natural frequency which is usually well in excess of 300 Hz and therefore not usually a problem since they ordinarily lie beyond the frequency measuring range of interest. When the frequency of tire rotation or any harmonics thereof coincide with one of these natural frequencies, the machine will resonate. As a consequence, the amplitude of force applied to the load cells at or near such frequencies could increase by a factor as high as 50. Thus, due to mechanical resonance a one pound force generated by the tire under test might transmit as much as fifty pounds to the load cell assembly thereby significantly degrading the accuracy of measurements made at or near a resonant frequency of the test machine.
The prior art has been to address the problem of mechanical resonance of the test machine by stiffening the machine in attempt to raise any significant resonant frequencies above the highest measuring frequency of the machine, i.e., above not only the fundamental tire rotational frequency but also above the highest harmonic thereof to be measured. Such stiffening has generally been attempted by constructing the machine of stiffer materials, and/or improving its bracing.
The tire industry continues to press for machines capable of measuring harmonics of higher and higher order. However, materials are not available to increase machine stiffness and therefore, resonant frequencies, indefinitely. While the stiffness of such a machine might be increased using the prior art techniques described above so that its lowest significant natural frequency approaches 150 Hz, such limits are simply not adequate to meet the goal of accurately measuring harmonics up to 300 Hz or even higher.